The surge limit of a compressor denotes a transition from an aerodynamically stable to an aerodynamically unstable operating mode, stability being understood as meaning the flowing of a process fluid in the provided flow direction. Considerable backflows occur with the start of the surging in a compressor, which backflows cause pressure fluctuations and temperature increases which already lead to damage after a comparatively short operating duration in this state. The reaching of the surge limit can be discerned clearly, even for an inexperienced operator, since the pressure fluctuations cause considerable oscillations, the noise of which exceeds the level of usual operation.
The operation of a turbocompressor therefore requires measures which avoid the reaching of the surge limit and restrict the state of surging to time periods which are as short as possible.
Accordingly, the regulator of a compressor always keeps a defined safety gap from a surge limit of the operating map. In addition, a surge limit regulating valve is provided as a rule which makes it possible to lower the pressure at the outlet of the turbocompressor if a defined gap from the surge limit is undershot or surging has already started. The state of surging or a tight closeness to the surge limit is characterized by a reduced mass throughput, mass flow or volumetric flow of the process gas to be delivered and with a corresponding pressure ratio over the compressor.
Conventional strategies for monitoring the surge limit or avoiding surging have already been described in DE 27 30 789 C2, DE 42 02 226 C2 or DE 43 16 202 C2. The methods there provide, for example, for an inlet pressure to be set in a ratio to an output pressure of a multistage compressor, and optionally for a volumetric flow and a temperature of the process gas also to be recorded along the compression path, preferably after intercooling, and for said measured variables for determining a surge limit of the machine to form the basis for the current operating point.
Experience shows, however, that determinations of this type are imprecise and these conventional monitoring methods require a comparatively great safety gap from the surge limit, which safety gap unfavorably restricts the available operating range of the turbocompressor. If this safety range is reduced, high loadings of the turbocompressor as a consequence of surging occur in certain operating states, with the result that the mechanical integrity is endangered.